1. Field of the Invention
The present invention relates to a current sensor which may be used in electrical power devices such as breakers, power relays for providing thermal, magnetic or short circuit protection functions.
2. Discussion of the Background
Current sensors comprising a primary circuit wherein an electric current to be measured flows, a magnetic circuit with an air gap and a secondary circuit delivering a typical signal from the current to be measured, are well known.
Certain types of current sensors provide a magnetic circuit in the shape of a torus around which a secondary winding is wound acting as a magnetic field detector. This layout may lead to significant manufacturing costs for the secondary winding in relation to the torus shape of the magnetic circuit, as compared to a cylindrical shape.
Other types of current sensors have a magnetic circuit with only one air gap. Accordingly, they are more sensitive to possible perturbations arising from external magnetic fields, and may thus require metal shielding around the secondary winding or a specific electronic circuit downstream from the secondary winding in order to limit the effect of these perturbations. Furthermore, the sensor""s dynamical range may then be reduced.
Finally, the document U.S. Pat. No. 5,617,019 describes a current sensor comprising a magnetic circuit consisting of two air gaps located between two U-shaped symmetrical parts and comprising two secondary coils placed either inside or around both air gaps. In this current sensor, the primary circuit only crosses the magnetic circuit once which makes the sensor less sensitive for low values of the primary current. Moreover, this sensor requires an information tap between the two coils, leading to a electrical connection point so requiring soldering, so that it may be used in an electronic processing downstream.
The object of the present invention is to produce a current sensor having good immunity to external magnetic fields, with reduced bulk, but especially covering a large adjustment range for the primary current and for which large mass production may be achieved in a simple way and in the most economical way possible.
According to the invention, the current sensor includes a primary circuit wherein an electrical current to be measured flows, a secondary circuit acting as a magnetic field detector and a magnetic circuit comprising two identical elements consisting of ferromagnetic material between which are located two air gaps with substantially equal thicknesses. This sensor is characterized by the fact that the secondary circuit includes two similar coils of a cylindrical shape, mounted in series, wherein each coil of the secondary circuit is positioned symmetrically relative to a median plane passing through the magnetic circuit air gaps and both coils are manufactured continuously by only one winding operation without any electrical connection between the two coils, which simplifies their mass production.
According to an advantageous embodiment of the invention, the magnetic circuit is made from a stretched profile with a circular cross-section. The primary circuit comprises an electrical conductor forming a cylinder-shaped winding of constant cross-section, encircling one of the two coils of the secondary circuit and symmetrically positioned with respect to the median plane, the conductor having a number of passages between the coils greater than 1.
The sensor is designed so that it may be adapted to several gauges of primary current by varying the number of passages of the primary current around the coils of the secondary circuit, the number of turns of the secondary circuit and the thickness of the air gaps, whilst maintaining a substantially linear and identical output signal, thus extending its use to the utmost and making it the most economical possible.